Flickering and architectural problems. The first is purely cosmetic, but is impossible to fix without making chances to the core protocol. The second means that an order of magnitude more work is required to add new functionality than what could be done with a more modern design.
Daniel Stone explains the problems with X11 in great details here: http://www.youtube.com/watch?v=RIctzAQOe44
obviously by ssh admin he means whoever administrates access to ssh, and would allow X forwarding in the sshd_conf file...
You are incorrect. X forwarding still requires giving your local host permission to the x server.
I don't know which distro you use, but usually that is enabled unless whoever administrates access to ssh disables it.